1. Field of the Invention
This invention relates to a ceramic multilayer circuit board comprising ceramic substrates of a low dielectric constant and conductors of a low resistivity, and particularly to a ceramic substrate for mounting LSI circuits.
2. Description of the Prior Art
Keeping pace with a shift of electronic circuits to higher density, multi-layer circuit boards wherein semiconductor elements are mounted on ceramic substrates have come to be used in large quantities. As these ceramic substrates, alumina ceramics have hitherto been used from the standpoints of thermal conductivity, mechanical strength, electrical insulation property and so forth.
Alumina ceramics have relative dielectric constants as large as about 9 and are produced by sintering at high temperatures of about 1500.degree. to 1650.degree. C.
It is known that the relative dielectric constant of a ceramic substrate affects the signal transmission speed of an electronic circuit and their relation is represented by the equation: ##EQU1## wherein t.sub.d is the transmission delay of electric signals, .epsilon.r is the relative dielectric constant of a ceramic substrate, 1 is the transmission distance of the electric signals and c is the speed of light. A large relative dielectric constant means a delay in transmission of signals corresponding to the largeness of the relative dielectric constant. Accordingly, the fact that alumina ceramics have large relative dielectric constants is a drawback for alumina ceramic substrates. Further, the fact that alumina ceramics require high sintering temperatures restricts the kind of applicable conductor materials to those such as tungsten, molybdenum and the like having high melting points, because alumina ceramic substrates and conductor circuits formed on the substrates are together subjected to sintering. Both tungsten and molybdenum as conductor material are resistant to sintering but have resistivities as large as 5.2 to 5.5 .mu..OMEGA..multidot.cm at room temperature. In circuit as 5.2 boards, when circuits are formed in high density, line widths are required to be narrower, whereby line resistances of circuits become larger, increasing voltage drop and delaying signal transmission speed. Hence, the use of these materials of high resistivities as conductor material is not desirable.
Thus, alumina ceramics having hitherto been used as substrates for multi-layer circuit boards have been disadvantageous in high speed transmission of circuit signals.